1. Field of the Invention
The present invention relates to a power management and control method of optimizing performance of power converters, in particular, the flyback converter.
2. Description of the Related Art
The use of wide-band-gap semiconductor devices and the demand for size reduction in today's external power supplies (e.g., adapters or chargers for laptops, tablets, mobile devices, game consoles, and printers) continue to drive substantial development and research efforts in high-efficiency and high-power-density power conversion technology. Generally, the size of a switch-mode power supply can be reduced by increasing the switching frequency because the sizes of passive components (e.g., transformers, input and output filters) can be reduced at a higher switching frequency.
As silicon-based devices approach their theoretical performance limits, further performance improvements of power supplies have become more difficult. However, emerging wide-band-gap devices (e.g., GaN-based and SiC-based devices) are expected to bring about future incremental efficiency improvements since these devices have a considerably lower gate charge and output capacitance than their silicon counterparts. Since wide-band-gap devices can operate at higher switching frequencies without deterioration in efficiency, such devices will enable further reductions in power supply size.
In low-power applications, the flyback topology is widely used due to its simplicity and lower cost. To achieve high efficiency at higher switching frequencies, switching losses have to be reduced. Reduction of switching losses can be achieved using various soft-switching techniques that utilize a circuit's parasitic components (e.g., leakage inductance of transformers and capacitance of semiconductor devices) to turn on a switch at a reduced voltage, or to turn it off at a reduced current. Specifically, under the zero-voltage-switching (ZVS) technique, the turn-on switching loss is eliminated by turning on a device at zero voltage and, under the zero-current-switching (ZCS) technique, the turn-off switching loss is eliminated by turning the device off at zero current.
An integral part of a flyback converter is the clamp circuit that processes the energy stored in the leakage inductance of the flyback transformer after the main switch is turned off. Generally, the flyback topology can be implemented with several clamp structures. Two common clamp structures are the RCD clamp and the active clamp, shown in FIGS. 1(a) and 1(b), respectively. In the RCD clamp, the energy stored in the leakage inductance of the flyback transformer is dissipated in the clamp resistor. In the active clamp, the energy stored in the leakage inductance is recycled and used to achieve ZVS turn-on of the main switch. It should be noted that, in the RCD clamp structure, ZVS turn-on of the main switch can be achieved only at an input voltage (VIN) that is lower than N times the output voltage (VO), where N is the turns ratio of the transformer (i.e., the ratio of the number of primary-winding turns to the number of secondary-winding turns). Thus, because of ZVS and leakage energy recycling, the active clamp approach generally exhibits better performance under high-load and high-input-voltage conditions than the RCD clamp approach. In contrast, at light-load conditions (i.e., when the flyback converter operates in discontinuous-conduction mode (DCM) and the active-clamp switch is turned on when the resonance between the magnetizing inductance of the flyback transformer and the circuit parasitic capacitances is almost completely damped out), the turn-on loss of the switch in the active clamp at a low input voltage (i.e., VIN<NVO) is greater than the loss reduction obtained by ZVS turn-on of the main switch. Furthermore, because the energy stored in the leakage inductance of the flyback transformer at a very light load is practically negligible, the performance of the flyback converter with an RCD clamp is better than that with an active clamp at very light-load and low-input-voltage (i.e., VIN<NVO) conditions.
It is thus desired to optimize the performance of the flyback converter over the entire line and load ranges.